The present invention generally relates to gas sensors and methods of determining the concentration of gaseous components, and in some cases, to sensors and methods for determining the concentration of oxides of nitrogen (NOx) in a gaseous atmosphere.
Sensors for determining the oxygen composition of gaseous mixtures, such as engine exhaust, are known to the art. For example, U.S. Pat. Nos. 4,272,329, 4,272,330, and 4,272,331 teach an oxygen sensor including a pump cell and a sensor cell, each having solid zirconia electrolyte and thin platinum electrodes attached thereto. The sensor cell and the pump cell, along with a ceramic tube, form an enclosed volume in which the ambient air establishes equilibrium by means of a leak opening in the ceramic tube. The pump cell is connected, by external circuitry, to an electrical input, while the sensor cell is coupled, by external circuitry, to electrical output measurement and control means.
The oxygen sensor taught by the '329 patent is operated in a steady-state mode whereby voltage is applied to the pump cell to electrochemically pump oxygen from the enclosed volume until a steady-state is reached wherein the rate of oxygen pumped from the volume is in equilibrium with the rate of oxygen diffusing into the volume through the leak hole. At steady-state, the oxygen partial pressure in the enclosed volume is reduced from ambient, causing an EMF to develop across the electrodes of the sensor cell. By adjusting the pump cell current to provide a constant sensor cell voltage, the pump cell current is linearly proportional to the percentage oxygen in the ambient atmosphere.
The oxygen sensor taught by the '330 patent uses a similar device operated in a transient mode to measure oxygen partial pressure. After ambient atmosphere of a desired oxygen partial pressure is established in the enclosed volume, the pump cell is activated to withdraw oxygen from the enclosed space. Reduction of oxygen partial pressure in the enclosed space causes an EMF to develop across the sensor cell. The first derivative of sensor cell voltage/time evaluated at or shortly after the initiation of a voltage drop is inversely proportional to the ambient oxygen partial pressure. The oxygen sensor may also be operated by pumping oxygen into the enclosed space and reversing the sign of the initial sensor cell voltage to determine the ambient oxygen partial pressure.
The oxygen sensor taught by the '331 patent uses a similar device operated in an oscillatory mode whereby a repetitive sequence of oxygen pumping currents flow to the pump cell in response to voltage inputs from the sensor cell. The pump cell withdraws oxygen from the enclosed space until the voltage drop induced at the sensor cell equals a predetermined reference value. The polarity of the pump cell current is then reversed to pump oxygen into the enclosed space until the sensor cell voltage reaches another predetermined reference value, at which time the pump cell current is again reversed and the cycle is repeated. With the magnitude of the pump cell current fixed, the period of oscillation is proportional to the oxygen partial pressure.
Sensors for determining the oxides of nitrogen (NOx) composition of gaseous mixtures, such as engine exhaust, are known to the art. For example, U.S. Pat. No. 6,344,134, discloses a method of measuring NOx concentration using a two-serial-space NOx sensor. The sensor includes a first pumping cell and a second pumping cell each comprising a solid electrolyte. In the sensor, a measurement gas space, a first space, and a second space communicate in series with each other. The method includes the steps of: pumping out oxygen from the first space into, for example, the measurement gas space, or pumping oxygen into the first space from, for example, the measurement gas space by action of the first pumping cell so that the oxygen concentration in the vicinity of a gas inlet of the second space becomes such that a portion of NO in the first space dissociates; dissociating residual NO and O2 in gas introduced into the second space from the first space by action of the second pumping cell; pumping out oxygen ions generated by dissociation of NO and O2 from the second space by action of the second pumping cell; and determining the concentration of NOx in the measurement gas based on signals (for example, pumping currents) issued from the first and second pumping cells.
Existing oxide-based gas sensors do not meet either performance or cost needs to address new environmental regulations, particularly with regard to engine exhaust. Improved oxide-based gas sensors are desired.